Neutering (Gonadectomy) the male dog
removes the source of circulating testosterone by removing the testicles, which
is presumed to be the major influence on observed changes in behavior following
gonadectomy.

Testosterone affects the male dog’s brain intensively at two
points in their development. The first is probably prenatally (Hart and
Eckstein, 1997) and the second is during sexual maturity. In the uterus,
testosterone can diffuse through the amniotic membrane and through uterine
blood flow. Once it reaches the male dog’s brain, it brings about
gender-specific structural changes that then relate to later development of
sex-typical behaviors. Remember that these structures start out female and
develop into male structures. These changes masculinize the male brain
structure. Prior to sexual maturity, male dogs display male-typical behaviors.
During the second significant surge of testosterone in the sexual adolescence
period of development, increasing testosterone levels continue to act on these
already established anatomical changes to again increase the frequency and
magnitude of male sexually dimorphic behaviors. “Testosteronetiters start to rise by the time the male pup reaches 4 to 5
months, where after testosterone levels reach a maximum at 10 months of age and
then fall to adult male levels by 18 months of age.” (Dunbar, 1999, p. 68) The
fact that the dog experiences two significant surges of testosterone, one of
which has significant effects on the anatomy of the brain prior to gonadectomy,
it is not surprising that gonadectomy does not have total control over sexually
dimorphic behaviors.

Ben and Hart at the University of California carried out one of the
most extensive surveys on the effects of gonadectomy on dogs, finding that at
least in intermale aggression, aggression was reduced by neutering in 60% of
cases with rapid reduction in 25%, and gradual reduction in 35% (Fogle, 1990,
p. 53). Neilson, Eckstein, and Hart, (1997) found that approximately 25% of
adult dogs that were aggressive toward humans or other dogs in the household
can be expected to have a 50 to 90% level of improvement after gonadectomy. A
50 - 90% level of improvement can likewise be observed in 10 to 15% of dogs
that are aggressive toward unfamiliar people or human territorial intruders
after gonadectomy. Neilson et al. also found that neither the age at which the
gonadectomy was performed or the duration the problem behavior existed for
affected the behavior after gonadectomy. Hart and Eckstein (1997) performed a
review of the research and literature on the effects of gonadal hormones on
objectionable behavior. They relay that gonadectomy affects sexually dimorphic
behaviors and that aggression toward other dogs and “dominance over owner” are
particularly sexually dimorphic. They point out that much previous research has
largely been based on guardian survey studies and has not experimentally
controlled for the placebo effect in that guardians may either instate some
form of changes that constitute behavior modification or they may imagine
changes that in fact did not take place or did not take place to the extent
they believed they did. Although this is a serious lacking in the validity of
the studies these same affects can be expected in real life gonadectomies
performed as part of behavior management plan. To the extent that the guardian
makes changes in their behavior or the environment that result in decreases in
aggressive behaviors simply as a result of having the gonadectomy performed
these same results can be expected in behavioral cases. A large scale German
study by Heidenberger and Unshelm (1990) found very similar results to those
described above.

Testosterone tends to promote greater reactivity in dogs. They
trigger a little quicker to aversive stimuli and respond a bit more intensely
and for slightly longer duration. Affecting the magnitude of aggressive
behavior could be particularly helpful in many cases.

Some general statements and recommendations can be formed from
these studies and notions.

·Neutering cannot be expected to reduce
aggressive behavior in all dogs

·Neutering will not always completely eliminate
aggressive behaviors

·When the neutering is carried out cannot be
expected to influence the rate or magnitude of changes in behaviors

·How long the problem behavior has existed does
not tend to affect the level or trend of change in behavior after neutering

·If an intact dog demonstrates aggressive
behavior neutering should be considered as an adjunct to other behavior
modification practices

·Early neutering is probably not helpful at
preventing aggression and may pose certain medical risks.

Spaying (ovariohysterectomy) is the surgical removal of the
ovaries and uterus, the source of estrogen and progesterone in female dogs.
Estrogenand
progesterone levels ebb and wane in cycles. The most significant influence
cycling fluctuations in estrogen and progesterones have on female dog behavior
is pregnancy related behaviors (female sexually dimorphic behaviors). “While
estrogen increases in the dog’s body for a short length of time, progesterone
remains in circulation, influencing the brain for two months after each estrous
and can have a dramatic effect on canine behavior. The most common behaviors
are those associated with pregnancy, nest building, guarding possessions and
milk production.” (Fogle, 1990, p. 54)

The most notable problem arises when the dog guards items
maternally. Other problems can involve irritability, conflict with other dogs
and energy reduction. Possessive guarding in intact females that occurs in
cycles is usually a hormonal guarding.

With a lack of testosterone to masculinize the brain in utero,
female dogs maintain their female brain structure. Both males and females start
with the basic brain organization for male and female behaviors. The phenotype
results from the probability or frequency with which the feminine or masculine
system is activated in very early development.

Sexual dimorphism is a matter of degree rather than an all or
nothing phenomenon. Most male dogs behave typically male and most female dogs
behave typically female in terms of sexually dimorphic behaviors. Some male
dogs display fewer or lower magnitude male sexually dimorphic behaviors and in
some cases display some female sexually dimorphic behaviors. Likewise, some
female dogs display fewer or lower magnitude female sexually dimorphic
behaviors and in some cases display some male sexually dimorphic behaviors.
Think of it as a continuum. One explanation for this is the basic similarity
and integration of behavioral systems of males and females of the species.
Another explanation is that the masculinizing effects of testosterone are a
matter of degree rather than a zero-sum trigger and that some individuals in
the uterus are exposed to greater or lesser amounts of testosterone.

It has been found (O’Farrell and Peachey, 1990; and also
discussed in Hart and Eckstein, 1997) that the only two behaviors affected by
spaying of female dogs was “aggressive dominance” toward guardians and
indiscriminate eating. In this case, the aggressive behavior increased after
spaying. This effect was only noted in female dogs ovariohysterectomized before
12 months of age, and that already demonstrated aggression. Female dogs
ovariohysteretomized after 12 months of age presented no risk of increased
aggression. Hart and Eckstein (1997) point out that female dogs are in a
progestational state for two months following an estrus period and spaying them
during that time creates a sudden removal of the source of progestins (which
tend to have a calming influence on animals). It is proposed that this removal
of progesterone may promote irritability or aggression in some individuals.

Another piece of this puzzle may involve androgenization of fetal
female dogs. It is suggested that either or both of two mechanisms may lead to
a slight masculinization of female brains in untero. If a female is positioned
between two males in utero, their brain may be masculinized by diffusion of
testosterone through the amniotic membrane. The other mechanism involves caudally
(closest to the tail of the animal) positioned males androgenizing rostrally
(closest to the head of the animal) positioned females in utero through the
fetal blood supply. For a good discussion of this, see Hart and Eckstein
(1997).

In a study by Kim, Yeon, Houpt, Lee, Chang and Lee (2005, in
press) it was found that female German Shepherd Dogs spayed between five and 10
months were significantly more reactive than an intact group. The intact group
was not exposed to a sham operation so even though the measures were taken well
after the surgery it is possible that the results are due to the surgical
procedure rather than the absence of ovaries and uterus. It is also possible
that the results are only reasonably generalizable to German Shepherd Dogs.
This study does suggest though that spaying may cause more reactivity in dogs.
Replication will be important.

Some statements and recommendations can be derived from the
above:

·Females should not be spayed within two months
after their estrus period to avoid sudden contrast between the presence and
absence of progesterone

·If a female demonstrates this consistent
“dominance” related aggression pattern prior to spaying and prior to 12 months
of age the risk of increased aggressive behavior must be balances against the
inconvenience of an intact female dog and potential health detriments of
remaining intact. Spaying can simply be delayed until behavior modification has
achieved progress

·Early spaying is likely not helpful at
preventing aggression and may pose certain medical risks.